Is there any good reason to avoid FOR loops without the final expression in JS?

Question

Is it a bad habit to write back loops like:

for (i = N; i--;) 

to access (N-1) to 0

If so, why? jsLint certainly doesn't like it.

Answer 1

There is no technical reason that this will not work. However, it clearly has problems with readability, as someone had an immediate “well, that won’t work!” reaction.

This is the problem that the jQuery team is struggling with - should we use new designs that save bytes due to clarity and maintainability. It really boils down to whether 1 or 3 bytes of savings are worth it:

 for(var i=9;i--;) var i=9;while(i--) for(var i=9;i>0;i--) 

In this case, probably not.

Answer 2

This is less readable, without your explanation it would take me a few seconds to figure out what the loop is doing. Why not just:

 while(i-- > 0) 

?

Answer 3

Did you consider readability? You can understand this very well, but other developers can get confused, as the for "idiom" parts are usually called :

 for ([initialization]; [condition]; [final-expression]) 

While a condition can be technically any expression, your version does not match this idiom, since the part of the “condition” that you use does more than just define the condition - it also shortens i .

Answer 4

"Bad habit of writing reverse loops:"

 for (i = N; i--;) 

This is a matter of opinion, but it is actually the opposite of while initializing, so in my opinion this is not a "bad habit". This is just a coding style.

The specification makes for parts optional to give the developer flexibility.

"jsLint certainly don't like it."

Who cares. You are not required to follow the opinions of jsLint.

Answer 5

The answers would be very subjective, I think. I don't think this is a bad habit, but I find it aesthetically uncomfortable. This can be expressed more elegantly, for example:

 for (i = N - 1; i >= 0; i--) { // do something here. } // And if it is really important that i should be 0 here // as it is in your original code. i = 0 

This code is easier in our brains when viewing a large amount of code that contains this.

Answer 6

Yes. There is a good reason. Readability.